Pattern mask generating method and apparatus

ABSTRACT

High-density multilayered integrated circuits are fabricated by first generating all photomasks for the respective layers concurrently during the same exposure operation. Since ambient conditions are identical during generation of these related masks, mask-to-mask misregistration is virtually eliminated. While photosensitive substrates are supported on a rotating turntable, a laser beam is incremented radially toward the turntable axis and modulated to expose selected portions of each of a series of contiguous concentric bands on each substrate sequentially. Thus, in generating a four-layered circuit, the four different photomasks for the respective layers would be generated by selective exposure of each mask in sequence within the same concentric band; then, after the beam is incremented radially, each mask would again be selectively exposed in sequence within a contiguous concentric band, etc. Alternatively, the laser beam is replaced by an electron beam; and a deflection yoke electrostatically deflects the electron beam to provide a series of chord-like contiguous scans as each substrate passes through the beam path. This provides rectilinear patterns on the masks without need for the programming necessary with a laser beam to convert X-Y coordinate data into polar coordinate data. The turntable is dynamically balanced in a unique manner; and clocking is precision monitored to enable precise registration of the masks capable, when photoreduced, to provide patterns with image elements as fine as five microinches.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for producing patternson photosensitive substrates, and relates more particularly to methodsand apparatus for generating the different photomasking patternsrequired for all respective layers of a particular high-densitymultilayered integrated circuit concurrently under identicalenvironmental conditions to minimize misregistration error.

Mask pattern generating machines currently commerically available employX-Y stepping of a substrate requiring over thirty hours to complete theexposure of a single mask pattern; and only one pattern can be made at atime. As circuit density increases, this time is expected to increase toover 120 hours per mask, further increasing the likelihood ofmisregistration errors that can result from changes in ambientconditions occurring from the time the mask for the first layer isstarted until that for the last layer is completed.

The most pertinent known prior art is U.S. Pat. No. 3,622,742. Thispatent shows and describes an apparatus in which a modulated laser beammachines, by vaporization, thin films on a plurality of substratesmounted about the periphery of a rotating drum. The beam passes througha lens that is oscillated to change the position of the beam focus tocompensate for change in depth of field as the flat-coated surface ofthe substrate rotates past the beam. The beam is also stepped in anaxial direction to impinge successively on different parts of thesubstrate during successive revolutions of the drum. Code plates arefixed to the periphery of the drum. Each plate has a photosensed timingtrack and is precisely located by index pins relative to its respectivesubstrate. Clocking pulses for each substrate are thus discontinuous;i.e., they are generated only while a laser beam is passing throughslots in a code plate, and hence cannot provide the clock continuitynecessary for a high-frequency clocking system required forhigh-density, fine-resolution circuits.

This cited patent suggests, however, that if desired, the substrates maybe mounted on the surface of a disk, rather than the periphery of adrum, and notes that coordinate transformation (such as employed in thepresent invention) would then be necessary to produce rectilinearpatterns. The patent further states that: "Normally all of the circuitsto be machined are identical, and if this be the case, the modulation ofthe beam is identical through each successive substrate that interceptsthe beam during one drum rotation." Even if this is construed to implythat the substrates may differ, there clearly is no teaching or evenremote suggestion that the substrates would be so related as tofabricate in the same exposing operation all of the photomasks necessaryfor making a particular multilayered circuit. Moreover, no prior art isknown which suggests making all such related masks concurrently in thesame exposing operation. Nor is there any teaching of a continuous clockarrangement to insure, with a permissible limited degree ofself-regulation, precision registration of patterns on the respectivemasks.

SUMMARY OF THE INVENTION

The principal object of this invention is to provide an improved methodand apparatus for fabricating photomasks for high-density, multilayeredintegrated circuits in a minimum amount of time and with minimallikelihood of misregistration error.

A plurality of substantially identical, flat substrates having aphotosensitive coating are supported on a flat turntable at preciselocations uniformly spaced apart both radially and circumferentially.The turntable is rotated at a substantially constant preselected angularvelocity. A laser beam or electron beam is incremented radially inwardtoward the turntable axis once per revolution of the turntable to scancontiguous narrow bands on each substrate sequentially.

A magnetic disk file, tape or other source of digital informationprovides a series of electrical pulses or bits, each representingsuccessive spots at which exposure of the photosensitive coating isdesired according to the particular pattern proposed on each respectivelayer of the integrated circuit. The light beam is modulated accordingto said series of bits to cause selective exposure of portions of theincremental bands of the photosensitive coating on each respectivesubstrate sequentially to provide an image on each substratecorresponding to its unique layer pattern, so that at the conclusion ofthe series of contiguous scans, the masking patterns for all requisitelayers will have been completely imaged in parallel under identicalenvironmental conditions.

Another object is to provide a continuous clocking arrangement embodyingmeans for correcting the beam modulation rate, and hence "printing"rate, should slight variations in turntable velocity occur betweenconsecutive rotations. According to this feature, two concentricmagnetic clocking tracks are provided. One track provides a base clock(e.g., 5000 bits per inch) which at the preselected velocity generates astring of pulses that are increased tenfold by an oscillator in a phaselock loop circuit to provide continuous high frequency system clockingpulses (e.g., at a rate of 32MHz). The other track provides one indexpulse precisely the instant each revolution of the turntable iscompleted. The phase lock loop circuit and another oscillator thatcontrols turntable velocity cooperate mechanically and electronically tomaintain synchronization between the base magnetic clock pulses, thehigh frequency system clocking pulses and the index pulse. The phaselock loop corrects beam modulation rate if the turntable velocity variesslightly from one revolution to the next. Circuitry hereinafterdescribed initiates a shutdown signal unless a precise preselectednumber of clocking pulses is generated between each successive indexpulse; failure to generate said preselected number of clocking pulsesindicates that there is a timing error, as a result of which image spotsgenerated in the scan just completed failed to register with thepredetermined precision relative to those in the preceding scan.

Still another object is to provide an improved and simplified means foraccurately and quickly achieving dynamic balance of the turntable, whichis critical to maintain interlayer registration within very closetolerance (e.g., five microinches).

These and other objects, features and advantages will become apparentfrom the following more particular description of preferred embodimentsthereof as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a photomask generating apparatusembodying the invention;

FIG. 2 is a schematic view, including certain control circuitry, of theapparatus shown in FIG. 1;

FIG. 3 is a perspective view of a lens holder assembly shown in FIGS. 1and 2;

FIG. 4 is a plan view of a dynamic balancing mechanism forming part ofthe apparatus embodying the invention.

FIG. 5 is a perspective view of a photomask generating apparatusconstructed according to a variation of the invention; and

FIG. 6 is a schematic representation of the manner in which the electronbeam used in the apparatus of FIG. 5 is electrostatically deflected.

DESCRIPTION OF PREFERRED EMBODIMENTS

As illustrated in FIG. 1, the apparatus embodying the inventioncomprises a heavy granite base 10 supported on an essentiallyvibration-free control table 11. Table 11 supports a hysteresissynchronous motor 12 that by a suitable damped coupling, such as amagnetic coupling 13, rotates a spindle 14 (FIG. 2) that supports anddrives a coaxially connected turntable 15. Turntable 15 is preferablyheavy enough, such as 150 lbs., to provide a flywheel effect to minimizechanges in rotational velocity from a nominal value, such as 600revolutions per minute.

A plurality (e.g., eight) substrate holders 16 are precisely affixed toturntable 15 equidistant circumferentially from each other and at equalradial distances from the rotational axis. Each holder registers in aprecise position a substrate 17 overcoated with a photosensitive coatingof essentially uniform thickness. The coating may be directly on asubstrate of transparent glass or on a chrome layer that is coated overtransparent glass.

Coaxially supported just above turntable 15, inboard of the substrateholders 16, is a clocking disk 18 having two concentric clocking tracks19, 20. Outer track 19 is a clocking track having permanently encodedthereon a series of uniformly spaced magnetized bits at a preselecteddensity (e.g., 5000 bits per inch); these bits are sensed by a magnetichead 21. Inner track 20 is an index track having a single bit encodedthereon that is sensed by a magnetic head 22, so as to provide a signalonly once every revolution of turntable 15. These magnetic heads 21, 22are carried by a stationary support block 23 in closely spaced relationabove the respective tracks 19, 20 on clocking disk 18. Disk 18 issecured so as to rotate in unison with turntable 15.

A suitable source, such as a laser 25, directs a beam of collimated,monochromatic, coherent light to a mirror 26 which, as illustrated,reflects it into a beam compressor 27, thence through a beam modulator28 and a beam expander 29 to a mirror 30, and thence into a lens holderassembly 31.

As best shown in FIG. 3, the beam is reflected from mirror 30 to amirror 32 and then vertically downward through an objective lens 33 viaan aperture 34 that is smaller than the beam diameter to allow foralignment and spot energy distribution losses about the periphery of thebeam for providing a very fine resolution spot. Lens holder assmebly 31carries mirror 32 and lens 33. A precision beam 38 anchored to granitebase 10 provides horizontal and vertical guide surfaces 38a, 38b thatextend in a direction parallel to an imaginary radial line extendingfrom the axis of turntable 15.

Laterally affixed to the side of assembly 31 is a bracket 39 having acentral vertical leg 40 at the ends of which are two integrally formedlegs 41, 42 that extend horizontally in opposite directions. As assembly31 is stepped radially inward, a capacitance probe 43 depending fromupper leg 41 senses vertical alignment of the laser beam dynamically,and a capacitance probe 44 projecting horizontally from central leg 40senses radial alignment of the beam. A capacitance probe 45 dependingfrom lower horizontal leg 42 senses the vertical position of thesubstrate 17 and holder 16 prior to pattern generation.

Affixed to surface X of holder assembly 31 is a piezoelectric block 48.An angle member 46 is affixed to the lower surface of block 48, andpiezoelectric block 47 is affixed to the vertical surface Y of angle 46.Objective lens 33 and aperture 34 are supported by a retaining member 35which is affixed to vertical surface Z of block 47. The foregoingarrangement enables precise vertical and radial alignment of theexposure beam. Each probe 43, 44 is set for a preselected gap (e.g.,0.022 inch). Piezoelectric crystal block 47 will expand and contracthorizontally under control of probe 44 as necessary to keep lens 33 andaperture 34 laterally aligned on a true radius of turntable 15. Crystalblock 48 will expand and contract vertically under control of probe 43to maintain lens 33 and aperture 34 at a constant preselected heightabove the substrate 17 to maintain spot size constant.

As shown schematically in FIG. 2, the photomask generator apparatusembodying the invention comprises a crystal controlled oscillator 50 forproviding pulses that are amplified by an amplifier 51 for controllingthe hysteresis synchronous motor 12. It further comprises an inputdevice, such as a reel 52 of magnetic tape, having magnetically encodedbit patterns thereon that correspond to the different photomask patternsdesired on the respective masks during each successive incremental scanby the laser beam. Said bit patterns operatively control the modulator28 to interrupt the light beam from laser 25 intermittently. Thisprovides a series of fine resolution light pulses that are so timed, inrelation to the rotation of turntable 15, as to cause light toselectively expose the photomask coating at desired image areas on thevarious substrates.

More specifically, the magnetic indicia on reel 52 is suitably processedin a conventional manner, as by a minicomputer 53. The data is gated outfrom computer 53 under control of suitable computer clock signals. Thedata preferably is in compressed form, requiring decompression bydecompression logic circuitry 54 before transmission to control logiccircuitry 55.

Circuitry 55 controls operation of modulator 28; it also controlsincremental movement of lens holder assembly 31 in a radial directionrelative to the axis of turntable 15, to thereby cause the modulatedlaser beam to scan successive contiguous incremental concentric bands ofthe photosensitive coated substrates 17 as each is rotated under themodulated beam. This incremental stepping of lens holder assembly 31 iscontrolled by a closed loop servo laser interferometer system (notshown) of the two-frequency beam type commercially available fromHewlett-Packard. Since the data is in the form of bits representing X-Ycoordinate positions at which a spot is to be exposed on each substrate,it must be converted into polar coordinate (R,θ) data by suitableprogramming, in a manner familiar to those skilled in the programmingart. The raster width of the laser beam scan is about an order ofmagnitude smaller than the thinnest spot or line of the desired maskpattern, for thereby enabling all curved lines to be straightened withsatisfactory resolution.

The electronic circuitry further comprises a servosystem including aphase lock loop circuit 56, a "divide by n" counter 57 and a comparecircuit 58. The phase lock loop circuit 56 is of conventional type. Itcomprises a phase detector (not shown) to which the magnetic clockpulses sensed by read head 21 are supplied at a preselected nominalfrequency (e.g., 3.2MHz) as determined by the density of the uniformlyspaced magnetized bits on clock track 19 and the preselected angularvelocity of turntable 15. The pulses from the phase detector aresupplied via a low pass filter (not shown) to a voltage controlledoscillator set to provide a series of high frequency (e.g., 32MHz)system clocking pulses in a line 59. Circuit 56 includes a "divide by n"counter (not shown) set to divide by 10, interposed in a feedback loop(not shown) connecting a branch of line 59 with the phase detector. Ifthe frequency of magnetic clock pulses sensed from clock track 19 ishigher than the frequency of the pulses fed back by the "divide by 10"counter, the low pass filter will go plus and cause the voltagecontrolled oscillator to speed up to bring the fed back pulse frequencycloser to that of the magnetic clock pulses; and vice versa. In thismanner, the beam modulation rate, and hence the "printing" rate, willmodify to compensate for slight variations in turntable velocity betweenconsecutive rotations.

Counter 57 is interposed between a branch of line 59 and the comparecircuit 58. It thus counts the system clocking pulses and divides thatcount by a preset number corresponding to the number of system clockingpulses that should occur per rotation of the turntable. Counter 57 willthus provide a signal to compare circuit 58 at that instant when saidpreset number of system clocking pulses have been counted. This signalshould coincide, within a selected "window", with the signal as sensedby index track read head 22 and also fed to compare circuit 58.

If this coincidence occurs, it verifies that the image spots generatedin the scan just completed register with the requisite precisionrelative to the image spots in the preceding scan. If these signals donot coincide, however, a timing error is indicated; i.e., the scan justcompleted has image spots unacceptably out of registry with those thatpreceded it, rendering further operation of the pattern generatorimpractical as the photomasks will be out of matching tolerance. In thisevent an error signal is generated by circuit 58 in a line 60, branches(not shown) of which are connected to computer 53, control logiccircuitry 55, drive motor 12, amplifier 51 and laser 25. This errorsignal will deactivate computer 53, control logic circuitry 55, laser25, and amplifier 51 and also actuate a direct-current braking system(not shown) for stopping the motor 12 and hence turntable 15. If no sucherror signal occurs, the control logic circuitry 55 will operate, aftera certain angular displacement past the index bit on track 20, toincrementally step lens holder assembly 31 radially inward a preselectedslight degree in readiness for the next contiguous scan. After a fullscan is completed, the lens holder assmebly 31 will be incrementedradially inward, and the apparatus is programmed to defer exposureduring this time interval.

As best illustrated in FIGS. 1 and 4, the apparatus embodying theinvention also comprises a simple dynamic balancing mechanism. Supportedon base 10 above turntable 15 is a weighing scale 65 (FIG. 1) having abalance beam 66 calibrated in terms of deviation of the substrate weightfrom a preselected value; this scale may either be in terms ofincreasing values from a known minimum value or in terms of a plus orminus deviation from a nominal value. In either event, thephotosensitive-coated substrate 17 is placed on a platform 67, balanced,and the scale reading noted.

As shown in FIG. 4, slug 68, housed in holder 16, has a calibrated scaleinscribed thereon which corresponds to the deviation terms inscribed onbalance beam 66 of scale 65. Slug 68 is radially positioned at a preciselocation at which a stationary pointer 70 is opposite and points to thenumber (deviation term) that had been noted on balance beam 66. Staticbalance of turntable 15 and holder 16 are corrected for, since the massof turntable 15 and holder 16 has been compensated for. All slugs 68 andassociated mechanism are equidistant radially with all slugs at the samedeviation number (e.g., 4). Hence, after noting the static balance ofeach substrate 17 as indicated on the balance beam 66, the operatormerely rotates a respective thumb wheel 69 as necessary to position eachslug 68 at the same deviation term as noted on balance beam 66.

To insure that the operator places a substrate 17 in each holder 16and/or that he has adjusted each thumb wheel 69 to correct for variationin substrate weight, a runout probe 71 is provided to sense dynamicrunout. This probe 71 is associated with circuitry (not shown) to supplyto line 60, via suitable OR circuitry, an error signal, similar to thetype heretofore described, for stopping motor 12 and turntable 15. Theapparatus will remain shut down until the deficiency has been rectified.This feature is essential to minimize runout to insure interlayer maskintegrity will be preserved.

After all substrates 17 are inserted in their respective holders 16,there will be a slight static imbalance--especially if there is amaximum variation in substrate weights. However, this static imbalanceis minimal. It is compensated to zero as the turntable is rotated to itspreselected velocity. These slight differences in weight are minimalfrom a static standpoint, although at the high preselected velocity ofthe turntable they are significant from a dynamic standpoint.

The invention as thus far described has been concerned with exposure ofphotosensitive coatings by a laser beam. It should be understood,however, that if desired the method and apparatus heretofore describedmay be modified slightly as indicated in FIGS. 5 and 6 to employ anelectron beam (instead of a laser beam) as the exposure energy source.

In the modified apparatus illustrated in FIGS. 5 and 6, structureidentical with that heretofore described in connection with the laserembodiment of FIGS. 1-3 will be designated by like reference numerals.The modified apparatus differs from that of FIGS. 1-3 in that laser 25and the other parts 26-33 associated therewith are eliminated; and lensholder assembly 31 is replaced by a source 100 of electron beam energywith which is associated a deflection yoke 101 of the typeconventionally used for television tubes for deflecting the electronbeam a limited degree. Also, since electron beam energy must be employedin an evacuated atmosphere, the apparatus must be suitably sealed by acover 102 over source 100 and a cover 103 for turntable 15; the areaenclosed by said covers being connected to a suitable vacuum source (notshown). The source 100 (such as an electron beam gun) is mounted on aY-slide 104 and incremented radially in substantially the same manner asdescribed in connection with the laser embodiment of FIGS. 1-3.

The electron beam embodiment has certain advantages. The deflection yoke101 is controlled in conventional manner by suitable hard wired logiccircuitry (not shown) so as to deflect the electron beamelectrostatically to provide a chord-like scan 105 (FIG. 6) as eachsubstrate 17 passes thereunder, returning to a normal curved path (i.e.,a concentric scan) between substrates, as indicated at 106. Thisdeflection yoke control desirably eliminates the special programmingneeded in the embodiment of FIGS. 1-3 to effect the polar coordinaterectification to convert X-Y data into polar coordinate (R,θ) data. Italso eliminates the need for maintaining a lens, like 33, constantlyperpendicular to the substrate and in critical focus, such as isrequired when using a laser beam because of its limited focal range.

The term "fine resolution energy medium," as used in the claims, istherefore intended generically to cover both a laser beam and anelectron beam, inasmuch as the invention, as broadly defined herein, maybe implemented with either.

It should be understood that this apparatus and method may be used tomake either positive or negative patterns. To make a positive pattern,as heretofore assumed, the X-Y digital data bits normally would be a "0"but be a "1" whenever the laser or electron beam is to be turned on toexpose the photosensitive coating. To make a negative pattern, the X-Ydigital data bits normally would be a "1" but be a "0" whenever thelaser or electron beam is to be turned on to expose the coating.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail,including but not limited to those above suggested, may be made thereinwithout departing from the spirit, scope and teaching of the invention.Accordingly, the apparatus and method herein disclosed are to beconsidered merely as illustrative and the invention is to be limitedonly as specified in the claims.

What is claimed is:
 1. A method for generating a set of photomasks for amultilayered integrated circuit, comprising the steps of:supporting aplurality of substrates with photosensitive coatings on a turntable atsubstantially equal radial and circumferential distances relative to theaxis of rotation; rotating the turntable at substantially a preselectedconstant velocity; providing an input medium having encoded indiciathereon representing that necessary to make all the masks for thedifferent layers of the multilayered circuit; directing a fineresolution energy medium toward the turntable and moving the mediumincrementally in a radial direction relative to the turntable to scansuccessive contiguous bands of each substrate coating sequentiallyduring successive rotations of the turntable; modulating the fineresolution energy medium under control of the encoded indicia so as tocontrol selective exposure of incremental portions of the coating oneach substrate in a series of contiguous narrow scans as the energymedium is incremented radially relative to the rotating turntable, toprovide a unique image pattern on each substrate corresponding to thatrequired for a respective one of the masks necessary for fabricating adifferent layer of the multilayered integrated circuit, so that themasks for all requisite layers will be completely imaged substantiallyconcurrently under identical environmental conditions and with minimalmisregistration during a series of continuous successive rotations ofthe turntable; and controlling the incremental radial movement of themedium and frequency of modulation with a continuous high frequencyclocking system having a closed loop feedback.
 2. The method accordingto claim 1, wherein the spot size of the fine resolution energy mediumis at least five times smaller than the smallest image element in anyimage pattern, and including the step of:coverting rectangularcoordinate positions of each substrate while it is supported in arespective predetermined position on the turntable, into polarcoordinates, thereby substantially to square up and render substantiallyparallel the opposite edges of the respective image elements to correctfor radial and concentric distortion due to rotation of the turntablerelative to the energy medium.
 3. The method according to claim 1,including the step of:dynamically balancing the turntable by adjustingthe respective radial positions of each of a plurality of predeterminedidentical masses, one associated with each substrate, to dynamicallycompensate for inertia changes due to the variations in weight of therespective photosensitive-coated substrates.
 4. The method according toclaim 1, including the steps of:weighing each photosensitive-coatedsubstrate, prior to supporting it on the turntable, to determine itsdeviation from a preselected weight, and providing at eachsubstrate-supporting location a slug which is movable radially relativeto the axis of rotation of the turntable by turning a respective thumbwheel to position the slug radially at a position corresponding to themagnitude of such deviation for dynamically balancing the turntable. 5.The method according to claim 1, wherein the controlling stepincludes:generating a continuous series of high frequency clockingpulses and a single index pulse during every rotation of the turntable;counting said clocking pulses; and generating a shutdown signal forinitiating a shutdown operation unless a preselected number of clockingpulses corresponding to the number that should be generated during everyrevolution of the turntable are counted between successive index pulsesfor terminating the mask generation process when clocking pulsefrequency variation would result in unacceptable misregistration of themasks for the respective different layers of the multilayer integratedcircuit.
 6. The method according to claim 5 wherein the controlling stepfurther includes:modifying the frequency of the clocking pulses, as withphase lock loop circuitry, to compensate for slight variations inturntable velocity during successive rotations of the turntable.
 7. Themethod according to claim 1 wherein the energy medium is an electronbeam, and including the step of:electrostatically deflecting the beam aseach substrate passes through the path of the beam to generate patternsin rectilinear format on the mask without need for programming.